Fluorides in a variety of forms have been demonstrated to be of significant value in reducing the occurrence of dental caries. Indeed, recognized authorities in the field of dental care have stated that there are "no bad fluorides" for this purpose.
Fluorides have been found to be best utilized for the prevention of caries when ingested, thus giving rise to the fluoridation of public water supplies. Fluorides are also of value when applied topically. A variety of topical fluoride treatments are provided professionally in the dental office. In addition, fluorides have been provided in commercial toothpastes and dentifrices. Of the fluorides available for topical application, stannous fluoride has proved to be the fluoride of choice due to the fact that the stannous ion combines with the naturally occurring phosphate in the enamel and dentin structure of the tooth to form stannous fluorophosphate which serves as a protective coating on the tooth surface.
Efforts have long been made by the leading commercial toothpaste marketers to make use of stannous fluoride as the source of "dentifrice-fluoride". Recently, however, the leading dentifrice producers have abandoned this effort, due primarily to the cost factor since stannous fluoride is several times as expensive as other available fluorides for the purpose. Moreover, the inherent instability of stannous fluoride in the presence of moisture and its reactivity with abrasives commonly used in dentifrices have contributed to the widespread abandonment of its use. It is difficult, if not impossible, to avoid the exposure of the stannous fluoride to moisture and to the abrasives present in a dentifrice which have a negative effect on the stability of the stannous ion.
Thus, efforts have increased to make stannous fluoride available in a stable and efficacious form in applications other than in toothpaste and dentifrices. For the stannous ion to be of value it must be freely available and not in combination with or locked into other chemical combinations as well as stable. Concentrations of stannous fluoride at a level of 0.1% available stannous ion have been demonstrated to be of value. A 0.4% stannous fluoride preparation has most frequently been demonstrated to be the concentration of choice in the treatment of dental caries.
While topical applications are frequently performed in the dental office there is also a need for follow-up daily application and use by the patient. Thus, "home-care" or "patient-care" availability is desirable. For this purpose a gel with the requisite viscosity to accommodate toothbrush application is the accepted marketable form.
Researchers recently discovered that stannous fluoride is relatively stable in anhydrous glycerin. However, glycerin solutions of stannous fluoride do not lend themselves to topical application to teeth because of their low viscosity. A variety of thickeners have been incorporated in glycerin solutions of stannous fluoride in an effort to increase the viscosity and, hence, the residence time of the composition on teeth when topically applied.
However, as in the case of dentifrices or toothpastes containing moisture and abrasives, the thickening agent used in preparing gels often contributes to the instability of the stannous fluoride. Not only is the choice of thickening agent critical to the stability of the available stannous ion, the technique in the process of making the gel product has been found to be of prime importance. In order to accommodate the demand for a "home-care" type gel, the product must be capable of being produced in quantities sufficiently large to make it economically feasible; i.e., "commercial-size batches" from which smaller consumer-size packages may be formulated.
Thickeners such as sodium carboxy methyl cellulose react with stannous ion thus contributing to the instability of the product. Hydroxyethylcellulose has been suggested for use as a thickener. Careful preparation of a gel under rigorous laboratory conditions employing hydroxyethylcellulose has yielded stable preparations. Under such controlled conditions, factors which contribute to instability are easily kept at a minimum.
Attempts to prepare commercial-size batches of gel with hydroxyethylcellulose economically, however, have consistently met with failure. The processing, mixing, temperature, apparatus, etc. requirements of economically feasible large batch gel production techniques adversely affect the stability of the stannous ion/hydroxyethylcellulose/glycerin system.
It is an object of the present invention to provide a method for the production in commercial-scale size batches of a stable gel containing stannous fluoride, glycerin and hydroxyethylcellulose which is stable over prolonged periods of time against deterioration.
It is a further object of the invention to provide a commercial-scale size batch of a stable gel containing stannous fluoride, glycerin and hydroxyethylcellulose.